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Origin of Solar System

The Universe is just too vast and beyond our imagination. We could be very confident that sun is really a very big part of universe but on the contrary the sun and the solar system where we live in is just the tiny part of Milky way galaxy. During the origin of universe came the galaxy formation, so came the star formation and later planets were formed. The earlier astronomers started the speculations with sun as they were very much familiar with it. This study was called cosmogony but this did not give out some great results as there was only one sample for study. Hence they started giving out the ideas with a name Origin of solar system.

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Solar System Theory

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We know that solar system consists of sun, moon, planets namely Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. This system was formed around 4.6 billion years ago due to the gravitational collapse of a part of the molecular giant cloud. The collapsed masses got concentrated at the center as the Sun but the rest mass acted as a Protoplanetary disk of which the planets, natural satellites like moon, asteroids and other bodies were formed. This is Nebular hypothesis and was first developed by Immanuel Kant, Emanuel Swedenborg and Pierre-Simon Laplace in 18th century. It also says that within the span of 10 billion years there would be no where any solar system existing.

Nebular Hypothesis

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This theory states that the whole solar system is just a large cloud of gas that gets collapsed into itself due to its own gravity. Here the rotating disk gets formed with a large concentration at center. Within the disk called the protoplanetary disk the planets get formed and at center the Sun formation takes place. Here are the four steps given in the diagram
Nebular Hypothesis

The nebular hypothesis states that the solar system started as a large mass of hot gases that contracts and compresses due to its own gravity. A nebula is an expansive, low-density cloud of gas and dust. The hypothesis states that angular momentum pulls the nebula into a disc, with the heaviest concentration of matter in the center. The center becomes enough dense enough to trap the infrared radiation that normally radiates away, thereby heating the center. The rotation of the disk splits off the newly formed proto-sun the center and forms rings that contract into planets. 

Evidence of Nebular Hypothesis

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The Kant's idea and Laplace's work helped gave out lots of evidence for Nebular hypothesis which are given as below:

• Kant's idea says that all planets revolve around the sun from west to east and the inclinations i of their orbits with respect to the central invariable plane of the solar systems are small. They range from $0.4^{\circ}$ for jupitor to $0.7^{\circ}$ for Neptune, $0.9^{\circ}$ for saturn, $1.1^{\circ}$ for uranus, $1.7^{\circ}$ for the earth and mars, $2.2^{\circ}$ for venus and $6.3^{\circ}$ for Mercury. For asteroids it has a mean value of $8.6^{\circ}$ and for pluto it $15.5^{\circ}$. If we consider a sin i where a is mean distance of planet from the sun, it is less than $0.4$ AU. Thus all the materials of solar system is restricted within a disc of thickness of $0.8$ AU which is having only $1$ percent of diameter of about $80$ AU. Similarly the sun and all planets, except venus and uranus rotate from west to east inclined to central plane $29^{\circ}$. Most of the satellites of the planets revolve around the parent bodies from west to east in equatorial plane. Hence it can be said that angular momentum comes from a single source. But for a contracting cloud we can take $\alpha$ = $0.1$ the cloud becomes bulged at equator and flattened at poles and hence a stage would be reached when the centrifugal force would exceed gravitational force. 

• Laplace work also provided some scientific basis. A rotating cloud of gas has an angular momentum whose value is $0.4$ for a sphere having uniform density and $0.08$ for a sphere with a central condensation of $60$ times the average density that holds most stars like sun. 

Transit Method

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The transit rule is applied when the angular measurements are more accurate that the linear measurements. According to the rule, the error total in the latitudes and in the departure is distributed in proportion to the latitudes and departures of the sides. It is also said that the angles are less affected by corrections applied by transit method than by those by Bowditch's method.
The transit rule is
Correction to latitude of any side = Total error in latitude $\times$ $\frac{Latitude\ of\ that\ line}{Arithematic\ sum\ of\ latitudes}$
We have
CL = $\sigma$ L $\frac{L}{L_r}$
Where L = Latitude of any line, D is departure of any line, Lr is arithematic sum of latitudes, Dr is arithmetic sum of departure, $\sigma$ L is error in latitude.

Transit method is method commonly used in determining exoplanets.This opts for plant-star system where the orbit aligned such that as seen from the planet earth, the distance between earth and star. It is even possible to determine the surface atmosphere, planet size from the light curve. The NASA mission used the transit method to discover planets in the constellation Cygnus. 

Protoplanet Hypothesis

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The Protoplanet hypothesis is all about expressing the nebular hypothesis with fluid dynamics. The Milky way Galaxy contained gaseous hydrogen molecules and dust particles that served as a condensation nuclei for the hydrogen. As the gas condensation continued, the gravitational attraction between the masses compacted the cloud into rotating disk that contains a proto-sun, a dense inner region, and a less dense outer region. The inner region formed the small planetisimals that congealed to form the terrestrial planets. The planets cooled and a crust formed on Earth.

Protoplanet Hypothesis

gaseous outer planets formed from the outer region of the rotating disk. The outer edge of the gaseous disk formed eddy currents that built up large masses of gas. The large masses condensed and their growing gravitational forces attracted more protoplanets to become even larger. The protoplanets eventually congealed to become huge gas planets. 

Formation of The Planets

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There are two ways for a matter in the solar nebula to organize. The bulk of matter formed the protosun via a gravitational instability that occured in a rarefied gaseous medium. The planets formed by a gradual process of accretion that started from dust and icy grains. The first step in the formation of planets is the formation of solid material by condensation. In the inner solar system solid consist of high-temperature condensates, "rocky" material such as that in the meteorites. In the outer solar system the temperature is lower and solids are composed of "icy" material such as that preserved in comets. The initial growth of dust grains from atomic size to planetisimals (1 km) is rapid and likely to complete within 104 yr. The growth from small bodies into planets involves five steps:
  1. Accretion of planetismals to the planetary embryo size
  2. Increase the eembryos by feeding o available planetisimals
  3. A slower growth and agglomeration of embryos into cores of present planets
  4. Growth of cores to critical size that is massive enough to accrete a gaseous envelope
  5. Termination of growth process due to tidal interactions between massive planet and nebula.

Pre-Solar Nebula

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Presolar nebula is the nebula formed out of slowly rotating interstellar cloud.It has gravitational energy very less and has diameter between 7,000 and 20,000 AU. The proplanetary disk is observed in a star forming region in the Orion nebula at a distance of about 1,600 Lyr. As it collapsed, it started rotating faster as per angular momentum conservation. Most mass accumulated at the center. Due to this rotation, the nebula flattened in to protoplanetary disk of about 200 AU and in the central region, a protosun evolved.Due to the nuclear fission the dust cloud formed the planetisimals and due to the collisions came out the planets. 
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